It is well known that biological materials and organisms fluoresce under ultraviolet (UV) light irradiation. UV light in the 380 nanometer (nm) wavelength range, for example, excites biological metabolic products such as Nicotinamide adenine dinucleotide, which is a coenzyme found in most living cells, and flavins, a group of organic compounds based on pteridine, to fluoresce. Higher energy UV, such as light in the 260 nanometer wavelength range, excites proteins. Since vegetative and spoor forms of bacteria contain these biochemicals, the bacteria will also fluoresce when irradiated with UV light. If these biological molecules are present in an aerosol exposed to UV light, they will also scatter part of the excitation light. Both the fluorescent and scattered light can be detected using existing light detectors such as a Photomultiplier Tube (PMT). The presence of both the fluorescent and scattered signal can be used to detect the presence of biological materials or organisms.
Some devices exploit this phenomenon to detect biological aerosol. These detectors typically use a pump to pull in ambient air containing the biological aerosols into some optical interrogation volume. An irradiative UV light, typically from a laser, light emitting diode or xenon lamp, is directed to the particles. The presence of biological particles in the optical interrogation volume will produce fluorescent and scattered photons. These photons are detected using a PMT or equivalent optical detector to produce an electrical signal. The relevant and absolute magnitude of this detected signal can be used to determine the presence of a biological particle. Since differing excitation wavelengths may be used to detect different classes of biological molecules, the excitation wavelength can be chosen to detect specific classes of biological molecules such as proteins, flavinoids, and metabolite products.
A biological aerosol detector of the type described above is described in more detail in U.S. Pat. No. 7,375,348, which is incorporated herein by reference.
Biological aerosol detectors are typically placed in locations where the potential for a biological attack or industrial accident exists. Under operating conditions, these detectors can be expected to continuously collect data for long periods of time, such as days, weeks, or months. The data is expected to consist largely of background signals and events, and to reflect the absence of signal events that indicate the presence of biological particles. To be effective under such operating conditions, biological aerosol detectors must be able to accurately and efficiently detect biological particles in an aerosol sample even when the particles are only present for relatively short periods of time within much longer observation periods.
Biological aerosol detectors typically detect the presence of particles using some form of peak detection algorithm. Peak detection algorithms, as their names imply, detect peaks in a data stream that rise above a background signal level, and that are indicative of signal events. There are a number of challenges in applying existing peak detection algorithms to biological aerosol detection. Photon detectors are susceptible to background noise, there is significant variability across detectors even when the detectors are from the same manufacturer and are of the same model, and detectors drift over time, temperature, and humidity conditions.
Accordingly, what is needed is a method of detecting biological aerosols using a photon counting technique that possesses a greater level of noise immunity, compensates for the variability across detectors, and mitigates detector drift. Also needed is a method to prevent over counting of biological particles that would otherwise result when photon signals produced from a single particle are distributed into more than one sampling bin.